Hi,
> On Mar 16, 2023, at 12:53 PM, Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Thu, Mar 16, 2023 at 04:38:41PM +0000, Qing Zhao via Gcc-patches wrote:
>>> NO. We have this debate every few years and such.
>>
>> So, what’s the major reason we keep the default that is not IEEE754
>> compliant from the beginning?
>
> It is compliant. fusedMultiplyAdd is a standard IEEE 754 operation,
After checking the IEEE754, IEEE standard for Floating-Point Arithmetic,
fusedMultiplyAdd is clearly documented:
"
fusedMultiplyAdd: The operation fusedMultiplyAdd(x, y, z) computes (x×y)+z as
if with unbounded
range and precision, rounding only once to the destination format.
"
> and
> C explicitly allows contractions.
However, I failed to locate where in C standard the fusedMultiplyAdd is allowed.
I got the draft version of C89, C99 and C11 from the following links:
• C89 – Draft version in ANSI text format:
(https://web.archive.org/web/20161223125339/http://flash-gordon.me.uk/ansi.c.txt)
• C89 – Draft version as HTML document:
(http://port70.net/~nsz/c/c89/c89-draft.html)
• C99 – Draft version (N1256) as HTML document:
(http://port70.net/~nsz/c/c99/n1256.html)
• C11 – Draft version (N1570) as HTML document:
(http://port70.net/~nsz/c/c11/n1570.html)
In both C99 and C11, I Only found the following section:
http://port70.net/%7Ensz/c/c99/n1256.html#7.12.13:
====
7.12.13 Floating multiply-add
7.12.13.1 The fma functions
Synopsis
1
#include <math.h>
double fma(double x, double y, double z);
float fmaf(float x, float y, float z);
long double fmal(long double x, long double y,
long double z);
Description
2 The fma functions compute (x y) + z, rounded as one ternary operation: they
compute the value (as if) to infinite precision and round once to the result
format, according to the current rounding mode. A range error may occur.
Returns
3 The fma functions return (x y) + z, rounded as one ternary operation.
====
My question: is the above section the place in C standard “explicitly allows
contractions”? If not, where it is in C standard?
Another question to ARM port maintainers is:
Is fmadd instruction provided by Aarch64 architecture IEEE754 compliant?
Thanks a lot for your help.
Qing
>
> Jakub
>
